The present invention relates to a differential pulse code modulation (DPCM) system for compressing and subsequently reexpanding video signals utilizing digital coding.
Video signals typically have many repeated sample values, and there is little information in the transmission of the repeated values. Thus video bandwidth compression is commonly used to reduce the actual data transmission rate, particularly when the transmit bandwidth is limited or processing gain against an external noise jammer is a requirement.
One way in which messages having repeated sample values can be compressed is to transmit only the digitally encoded differences between successive sample values. This technique is known as differential pulse code modulation (DPCM). This type of system reduces both the time and cost of transmitting a signal in a narrow bandwidth.
A typical DPCM transmission system comprises a transmitter for transmitting a video signal and a receiver for receiving the signal, the transmitter including a DPCM encoding and decoding arrangement, and the receiver including a DPCM decoding arrangement for reconstructing the coded video signal.
In a DPCM encoder, the value of an image sample is predicted and the difference between the actual and the predicted value is quantized and transmitted. At the decoder, a similar predictor uses the transmitted values of the quantized difference signal to reconstruct the scanned image sample. The basic block circuit diagrams for a simple DPCM encoder and decoder are shown in FIGS. 1 and 2, described in more detail below. Modifications of such systems are described, for example, in U.S. Pat. No. 4,200,886 of Musmann et al., U.S. Pat. No. 4,292,651 of Kretz et al., and U.S. Pat. No. 4,386,366 of Mori.
In the encoder, an input digitized video signal A containing an N bit word per picture element (pixel) comprises the encoder input. A digital prediction value B is subtracted from the input signal A in a suitable subtractor circuit to form a difference signal E which is quantized by quantizer Q. Quantizer Q is selected to have a predetermined quantization characteristic consisting of L quantization levels each corresponding to a predetermined range of the input signal. Quantizer Q produces a quantized difference signal output. This output is transmitted to a suitable receiver and also connected to a predictor circuit to produce prediction value B. The predictor circuit typically comprises a Q.sup.-1 recontructor to reconstruct the difference signal D, an addition circuit to add signal D to prediction value B, a feedback multiplier to multiply the output of the addition circuit by a chosen feedback coefficient, and a latch clocked to produce one pixel delay in the prediction value B so that each incoming signal has a value B reconstructed from the coded output signal of the preceding signal subtracted from it.
At the receiver, the received signal is decoded and the recovered difference signal D is added to prediction value B in order to regenerate the quantized signal.
DPCM techniques of this type have up to now employed a lot of digital circuitry working at high speeds, typically up to 30 or more integrated circuits for carrying out the various functions of the encoder and decoder. These circuits may take up to 4 or 5 circuit boards, requiring a lot of space and having relatively high power consumption. Thus their use is limited where the space, weight and power available is restricted, for example in aircraft, satellites and the like. There is therefore a need for DPCM systems utilizing a minimum of circuitry.